Add more checks for onlyif / unless

These commands are supposed to be supported, but they are not tested, so
add tests to demonstrate that they work as intended.

spec/puppet-lint/plugins/check_unsafe_interpolations_spec.rb

@@ -33,19 +33,23 @@ class foo {
 
           exec { 'bar':
             command => "echo ${foo} ${bar}",
+            onlyif  => "${baz}",
+            unless  => "${bazz}",
           }
 
         }
         PUPPET
       end
 
       it 'detects multiple unsafe exec command arguments' do
-        expect(problems).to have(2).problems
+        expect(problems).to have(4).problems
       end
 
       it 'creates two warnings' do
         expect(problems).to contain_warning(msg)
         expect(problems).to contain_warning("unsafe interpolation of variable 'bar' in exec command")
+        expect(problems).to contain_warning("unsafe interpolation of variable 'baz' in exec command")
+        expect(problems).to contain_warning("unsafe interpolation of variable 'bazz' in exec command")
       end
     end